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Currently, expression does not support the "constant null" well in constant folding.
e.g. Sum(a, 0) actually always produces Literal(0, NumericType) in runtime.
For example:
```
explain select isnull(key+null) from src;
== Logical Plan ==
Project [HiveGenericUdf#isnull((key#30 + CAST(null, IntegerType))) AS c_0#28]
MetastoreRelation default, src, None
== Optimized Logical Plan ==
Project [true AS c_0#28]
MetastoreRelation default, src, None
== Physical Plan ==
Project [true AS c_0#28]
HiveTableScan [], (MetastoreRelation default, src, None), None
```
I've create a new Optimization rule called NullPropagation for such kind of constant folding.
Author: Cheng Hao <hao.cheng@intel.com>
Author: Michael Armbrust <michael@databricks.com>
Closes #482 from chenghao-intel/optimize_constant_folding and squashes the following commits:
2f14b50 [Cheng Hao] Fix code style issues
68b9fad [Cheng Hao] Remove the Literal pattern matching for NullPropagation
29c8166 [Cheng Hao] Update the code for feedback of code review
50444cc [Cheng Hao] Remove the unnecessary null checking
80f9f18 [Cheng Hao] Update the UnitTest for aggregation constant folding
27ea3d7 [Cheng Hao] Fix Constant Folding Bugs & Add More Unittests
b28e03a [Cheng Hao] Merge pull request #1 from marmbrus/pr/482
9ccefdb [Michael Armbrust] Add tests for optimized expression evaluation.
543ef9d [Cheng Hao] fix code style issues
9cf0396 [Cheng Hao] update code according to the code review comment
536c005 [Cheng Hao] Add Exceptional case for constant folding
3c045c7 [Cheng Hao] Optimize the Constant Folding by adding more rules
2645d4f [Cheng Hao] Constant Folding(null propagation)
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